1. Field of the Invention
The present invention relates to a power amplifier installed in wireless communication terminals and systems, and more particularly, to a power amplifier to which a mode capable of selecting a wide band frequency is applied.
2. Discussion of Related Art
Standards on wireless LAN services began to be discussed ten years ago as demands for various multimedia services in a near field wireless environment have continuously increased, and a market of wireless LAN products supporting up to 600 Mbps based on IEEE 802.11n platform is becoming increasingly active. However, a transmission speed Gbps or more is required to transmit an image of ultrahigh image quality without compression, and because performance of 802.11n cannot satisfy this, IEEE formed a IEEE 802.11ac group and conducted a standardization activity in November, 2008 in order to develop a standard on VHT wireless transmission technology, which is a next generation wireless transmission method. IEEE 802.11ac is a standard of next generation wireless LAN focused on simultaneous connection of multiple users and high performance support that surpasses that of Gbps grade and may provide services for blue ray and uncompressed ultrahigh image quality video in real time. Recently, demands for smart phone which employs WiFi is explosively increasing, and IEEE 802.11ac is becoming a core technology for a next generation smart phone. Meanwhile, recently wide area wireless LAN employing a frequency band under 1 GHz has been on the rise, and, for example, there are IEEE 802.af employing TVWS band and IEEE 802.11ah employing a 900 MHz band. The main aim of these developments is beyond the expansion of a smart grid and a wide area sensor networks towards an extended range WiFi service.
When a selection of a wide band frequency is needed, an output impedance matching network may be designed by a selective matching based on a frequency bands, or a multi-level and multi-band power amplifier may be designed using switching devices and varactors. However, both the output impedance matching network and the multi-level and multi-band power amplifier use switching devices to change an output, a frequency band, etc., and therefore the output, impedance matching, etc. may be affected by the characteristics of the switching devices.
Generally, a method of selecting a frequency band includes a transmission line transformer (TLT) and capacitors or varactors disposed on both sides of the TLT. This method additionally needs a transistor which is a switching device. However, the switching device is a passive device and therefore not only generates additional power loss but also has poor linearity.
Another method of selecting a frequency band used previously is a method of properly tuning a length of a transmission line transformer which is an output impedance matching network. For impedance matching, as the method of tuning a length of a transmission line transformer, there are two methods such as a method of using switches, using a thin metal layer in a semiconductor process, etc. as described above. The main problems with the two methods are that efficiency of a power amplifier is reduced and sizes of a transmission line and a transformer are increased. Generally, the sizes of the transmission line and the transformer occupies the most space of an inside area of a power amplifier chip.